The present disclosure relates to memory devices and programming methods for memory devices. More particularly, the disclosure relates to memory device programming methods that control the levels of a pass voltage and a local voltage in relation to a program voltage that increases during a program operation.
Over the past several decades, flash memory has become a commercially important form of nonvolatile memory. Like all nonvolatile memory, flash memory is able to retain stored data in the absence of applied power. Yet, flash memory is capable of being very densely integrated on a stored bit per unit area basis. Increasing integration density is enabled by shrinking design rules (i.e., constituent components are designed with smaller sizes and less inter-component spacing). Unfortunately, shrinking design rules lead to certain fabrication and performance problems with the resulting memory devices.
For example, design rules for contemporary flash memory continue to shrink, it becomes increasingly difficult to secure and maintain a so-called pass voltage window. A “pass voltage window” is an important performance consideration in flash memory and is critical to successful program operations.
During a program operation, a program voltage (Vpgm) is applied to a selected word line, and a pass voltage (Vpass) is applied to a non-selected word line. Under these conditions, a program voltage disturb may occur due to a low pass voltage. A program voltage disturb is a well known phenomenon in flash memory and arises when one or more program-inhibited memory cells within a plurality of memory cells connected to a selected word line (i.e., a word line receiving the program voltage (Vpgm)) are nonetheless programmed by the applied program voltage. This inadvertent programming of what should have been program-inhibited memory cells results in a program operation failure.
A pass voltage disturb is a similar type of programming failure. It arises, however, from a high pass voltage. That is, a high pass voltage is applied to memory cells within a plurality of memory cells connected to a memory cell string resulting in the inadvertent programming of memory cells and a program operation failure.
Thus, a pass window voltage may be understood as a voltage range bounded on one end by a pass voltage level so low that a program voltage disturb occurs and on the other end by a pass voltage level so high that a pass voltage disturb occurs. Clearly, the overall reliability of the program operation is enhanced by obtaining the widest possible pass voltage window.